In the electronics industry, ever-present goals are to miniaturize integrated circuit chip packaging and to minimize the lengths of interconnections between chips. Multi-chip modules were designed to address these goals. Multi-chip modules package and interconnect a plurality of chips in a functional relationship. The advantages of multi-chip modules are primarily in terms of packaging density and operational speed. The main disadvantage involves the cost of manufacturing such modules.
Among the reasons for the high cost of multi-chip modules is the relatively low manufacturing yield of the modules. As the number of integrated circuit chips within a package increases, the likelihood of a defect within the package also increases. Typically, testing of the individual chips prior to interconnection within a multi-chip module is not possible. Later detection of a defective chip within a module may result in the entire module having to be discarded.
An interconnection technique which permits pretesting of chips is referred to as "tape automated bonding". This fabrication procedure utilizes a continuous insulated tape which is similar to photographic film to provide a planar substrate for chips that are attached to individual sections, or frames, of the tape. A spiderlike metal pattern of conductive traces is etched on each frame. The traces may either "fan out", i.e. radiate from the center of the frame to the four edges, or may be four sets of parallel lines, with each set extending perpendicularly from one edge of a chip. The chip is carefully aligned over the center of the frame so that the contacts of the chip are precisely located at corresponding conductive traces in the central portion of the frame. The chip is then attached to the tape automated bonding frame. This connection of the chip contacts to the inner portion of the frame is referred to as "inner lead bonding".
After the inner lead bonding has been performed, the integrated circuit chip may be tested. The chip can be thoroughly exercised electrically. Following testing, the outer leads of the frame are microbonded to pads on a substrate. The attachment of the conductive traces of the frame to the pads of the substrate is referred to as "outer lead bonding." The substrate is that portion of the multi-chip module which permits mounting of more than one lead frame for electrical communication between the chips supported by the frame.
While tape automated bonding provides an important improvement in the interconnection of integrated circuit chips because it allows pretesting of chips, rework and repair of a circuit having a number of chips is still problematic. A chip having a size less than onehalf inch in length may have more than 400 connection sites. Removal of a single chip from a larger circuit typically requires disconnection of each of the more than 400 outer leads from the substrate connection sites. Desoldering of the lead frame from the substrate connection sites can be a labor-intensive task.
U.S. Pat. No. 4,806,503 to Yoshida et al. teaches a method of replacing integrated circuit chips interconnected within a multi-chip module by use of tape automated bonding. The method of Yoshida et al. is to cut the conductive traces of the lead frame at the centers of the conductive traces. That is, the outer lead bonds are left intact to support the outer portions of the conductive traces. A replacement part having a lead frame with conductive traces sufficiently long to overlap the conductive traces left from the first-installed lead frame is then precisely aligned to allow bonding of the second lead frame to the conductive traces of the original lead frame. While the Yoshida et al. method does permit replacement of chips within a multi-chip module, the method requires a second set of microbonds. To minimize resistivity, it is preferred that the number of microbonds be minimized. Moreover, while bonding devices exist to secure lead frames to substrates, these bonding devices may not be dimensionally or functionally adaptable to microbond a first lead frame to a second lead frame.
It is an object of the present invention to provide a system and method for interconnecting electronic devices wherein the system and method allow testing of devices prior to final mounting and wherein the resulting assembly can be easily reworked or can be removed in its entirety.